The Public Switched Telephone Network (PSTN) is now almost completely digital. The "last mile" between the central office (CO) and a telephone set, also known as the "local loop", is the only analog portion of the telephone network. The central office and backbone of the telephone network is completely digital.
Digital signals on the US network are transmitted over a variety of carrier media, including T-carrier systems such as T1 lines. T1 lines operate at 1.544 Mbps and carry a maximum of 24 64-Kbps voice channels. T1 (and T3) lines utilize a sampling technique called Pulse Code Modulation (PCM) for conversion between analog and digital signals. The analog signal is sampled at 8 kHz and quantized with an 8-bit/sample quantizer to form digital information that allows the original analog signal to be recreated at a receiving location. A connection between an end user on a local loop and one on the digital network can be referred to as a "PCM channel".
T1 lines use a framing structure in which each of 24 voice channels is allotted 8 bits. The T1 frame consists of 193 bits, where the extra bit is appended to signify the frame boundary. From the point of view of a single voice channel, frames consist of 8 bits. A frame viewed from the perspective of a single channel can be referred to as a "PCM frame" or a "slot".
It is possible to send data into the digital network through the local loop as well as analog signals. In order to send data, it is necessary to format the data to fit the PCM channel constraints. An example sequence of PCM frames 10 is shown in FIG. 1. For digitized analog signals, each slot S holds one 8-bit sample of the analog signal. For data, the modulation scheme maps bits to symbols and symbols to a constellation. The sequence of constellation points resulting from the data is converted to an analog signal and transmitted through the local loop. The network treats this signal as it would any analog signal.
Each frame can contain network signaling information as well as the digitized samples. The signaling information is used to indicate information such as the status of a call, or whether a phone is off the hook. An in-band signaling technique called Robbed Bit Signaling (RBS) is used to carry signaling information over a T1 line. Because there is no spare bandwidth to carry signaling information, RBS periodically "robs" one bit from a particular frame. This bit is then used for signaling information. A standardized RBS for individual T1 lines robs 1 bit of every sixth frame. This bit is the least significant bit (as shown by 12 and 14 of FIG. 1). Therefore every sixth sample of voice encoding contains 7 bits of voice data and 1 bit of signaling information. The degradation caused by RBS to voice samples is fairly minimal.
It is convenient to describe a PCM frame stream as a continuous periodic repetition of "RBS Frames" 16. Each RBS frame 16 is composed of six slots S. Each slot contains one 8-bit PCM codeword (referred to as an "octet"). Once a call is established, the specific slots affected by RBS remain fixed for the duration of the call. For the example shown in FIG. 1, slot S6, slot S12, etc. are affected by RBS.
Different carriers in the network can choose a different slot out of the RBS frame to rob. As the sequence of slots passes through the network, multiple slots within an RBS frame can be affected. The pattern of robbed bits observed at the output is identical from RBS frame to RBS frame.
Because RBS robs information, it causes problems when data are being transmitted. As previously described, the loss to voice (analog) information is fairly minimal. However, when digital data are being transmitted or received, such as by a modem, the periodic loss of a data bit causes continual errors. From a modem's perspective, RBS increases quantization noise for certain slots in the upstream data by stripping off one bit 12, 14 of information that is normally used to convey the least significant bit of the PCM code carrying the received upstream signal amplitude at the central office codec. In other words, RBS imposes a constraint on the system.
In order to cope with RBS, PCM modems such as those that conform to ITU-V.90 must impose a more restrictive design constraint when selecting constellations for slots affected by RBS. The design constraint typically reduces the total number of constellation points, or equivalently, increases the minimum code point distance an RBS affected slot can reliably support. PCM modems must select different constellation points for slots affected by RBS.
The following example illustrates how the V.90 Standard copes with RBS. In V.90, the first symbol of TRN2d is designated as slot 0 of the RBS frame. The length TRN2d and all training sequences thereafter are constrained such that the first symbol of each data-mode frame also falls on slot 0 of the RBS frame. This is accomplished by extending each subsequent transmitted field's length to a symbol span that is an integer multiple of six octets. Therefore, in data mode, the RBS slot of each symbol in the data frame, or equivalently the position of the V.90 downstream data-frame relative to the network RBS frame, is haphazardly decided by the timing of the digital PCM modem transmitter and is fixed after the start of TRN2d for data mode and for all subsequent rate negotiations.
Similarly to ITU-V.90 like PCM modems, the design of PCM upstream modulation schemes must be constrained as a result of RBS. Because a PCM modulation scheme like the one disclosed in U.S. patent application Ser. No. 09/234,451 filed on Jan. 20, 1999, imposes an additional design constraint due to trellis coding, the V.90 method for dealing with RBS cannot be utilized efficiently with a PCM upstream modulation scheme. Any coding scheme that places a restriction on a symbol based on previous symbols may conflict with the RBS restriction, reducing efficiency. Thus, both the data frame position and network RBS frame position impose constraints on the constellation points and mapping parameters that may be successfully employed for specific slots within the data frame. In the disclosed PCM upstream modulation scheme, additional constraints are imposed on the possible constellation sets and mapping parameters so the power constraint can be satisfied for trellis modified data-frame slots 3, 7, and 11.
Unfortunately, the solution utilized by V.90 to overcome RBS constraints does not work for PCM upstream modulation schemes. As an example, the upstream data frame for V.92 is 12 slots long. Fixing the first symbol of each upstream data frame to coincide with the first symbol of the training sequence transmitted by the analog PCM modem does not provide a satisfactory solution because together the RBS and trellis coding constraints can over-constrain the transmit constellation. Thus, there is a need for a method that can be utilized with a PCM upstream modulation scheme that allows a digital modem to adjust the relative phases of the network RBS frame and the upstream data mode frame so the RBS affected slots coincide minimally with the trellis modified symbols. If the RBS affected slots coincide with slots affected by encoding, the system has a low amount of flexibility, resulting in degradation of performance.